The present invention relates to analog to digital (A/D) converters, and more particularly to an A/D converter in which an asynchronously swept thermometer code is used to perform the A/D conversion.
Conventional A/D converters employing parallel architectures, such as flash converters, are widely used today in electronic circuits. In such converters an analog input signal is transformed into a digital binary thermometer code which is then translated into a binary code using a conversion table. The thermometer code is used to generate an analog ramp voltage whose value is compared to an analog input voltage to selectively determine when the analog to digital conversion occurs.
Conventional A/D converters of the type just described suffer from several drawbacks. First, they typically employ a high speed clock which requires several clock cycles, and thus a relatively lengthy period of time, to synchronously sample the signal to be digitized before the analog to digital conversion occurs. Second, such converters also typically employ a plurality of voltage comparators for comparing the analog input voltage to a plurality of uniformly spaced reference voltages. These comparators are relatively complex, occupying a large surface area and consuming a great deal of power. Finally, the random offsets of the voltage comparators introduce nonlinearity into the A/D conversion which impedes the accuracy at which conversion occurs at a selected analog input voltage.
An A/D converter employs a chain of buffers to generate an asynchronously swept thermometer code that is used to derive a ramp voltage that is compared with an input voltage to selectively determine a voltage at which D/A conversion occurs. When the ramp voltage equals the input voltage, the thermometer code existing at that time is 1) frozen by halting the propagation of bits through the buffers and 2) translated into a binary code using a conversion table to determine the amplitude of the digitized voltage.
Advantageously, use of the chain of buffers enables the thermometer code to be swept at a frequency that is lower than the frequency used in conventional A/D converters. Further advantageously, the use of single voltage comparator requires less power than conventional A/D converters, and it does not introduce nonlinearily into the A/D conversion so that an improved conversion resolution is attained compared to conventional A/D converters.